1. Field of the Invention
This invention relates to the production of glass with a reduced incidence of gas inclusions in the final product. More particularly, the invention relates to a method of acting on molten glass contained in a glass melting furnace to encourage the accumulation of gas inclusions at the surface of the molten glass and to provide such gas inclusions sufficient residence time upon the surface to promote their dissipation into the furnace atmosphere.
2. Discussion of the Technical Problem
In the making of a flat glass ribbon, it is desirable to withdraw a stream of molten glass from the exit end of a glass melting furnace which is substantially free of gas inclusions, or seeds. Gas inclusions remaining in the withdrawn stream of molten glass may pass downstream and enter the final product, e.g., a ribbon of float glass, to form a defect therein.
Commonly, heat is applied within a glass melting furnace in a manner which generates a pattern of thermally induced convection flows in the pool of molten glass contained therein, including an outlet end flow pattern, an inlet end flow pattern, and a spring zone therebetween. Generally, the portions of the outlet and inlet end flow patterns moving adjacent the bottom of the pool converge together at the spring zone, where the molten glass then flows upwardly toward the surface of the molten glass pool. The upper portions of the outlet and inlet flow patterns then diverge, with the upper portion of the inlet end flow pattern moving toward the inlet end of the furnace. The upper portion of the outlet end flow pattern moves toward the outlet end of the furnace, where a portion is withdrawn as throughput. These flow patterns may be beneficial because they promote proper batch melting and homogenization, however, they also may disadvantageously promote the incidence of gas inclusions in the final glass product, e.g., a flat glass ribbon.
More particularly, it is desirable to maintain molten glass within the flow patterns of the molten glass pool for prolonged periods of time in order to provide opportunity for gas inclusions within the molten glass to rise toward the surface of the pool and either dissipate into the furnace atmosphere or dissolve into the molten glass. There has been identified, however, a current of flow within the molten glass pool that has a minimum residence time within the furnace (hereinafter minimum residence time flow) which tends to inhibit the removal of gas inclusions from the molten glass pool. As represented in FIG. 3, in a conventional glass melting operation there is a substantially endless inlet end flow pattern 90 and a substantially endless outlet end flow pattern 94 generally circumscribed by a minimum residence time flow 88, which moves along the indicated path at a relatively great rate toward the throughput stream 96. Gas inclusions in the molten glass tend to rise toward the surface of the molten glass pool, thereby entering and becoming entrained in the minimum residence time flow 88. Due to the relatively high flow rate of the minimum residence time flow 88, such entrained gas inclusions are swept downstream into the throughput, rather than having sufficient time adjacent the surface of the molten glass pool to dissipate into the furnace atmosphere or dissolve into the glass. This effect increases the incidence of gas inclusions in the throughput stream, with a corresponding increased incidence of gas inclusion defects in the final product.
Techniques have been utilized to control patterns of glass flow within a glass melting furnace and/or to eliminate gas inclusions from the molten glass therein. For example, U.S. Pat. Nos. 1,631,204 to Hitchcock; 1,641,898 to Neenan; 3,771,984 to Demarest; 3,976,464 to Wardlaw, 3,989,497 to Dickinson et al; 4,023,950 to Glaser; 4,046,546 to Hynd; and 4,052,186 to Rhodes each generally teach physical barriers, e.g., skimmers, floaters, etc., which act upon the upper surface of a pool of molten glass in a glass melting furnace to affect flow patterns therein. Such barriers are limited, however, to providing a localized damming effect, permitting a minimum residence time flow to accelerate thereunder while retaining entrained gas inclusions.
U.S. Pat. No. 3,321,289 to Touvay generally teaches a rotatable baffle member which may be immersed within the pool of molten glass adjacent the floor of a glass melting furnace to alter flow patterns therein.
U.S. Pat. Nos. 1,744,359 to Brown; 2,688,469 to Hohmann; 3,244,496 to Apple et al; 3,498,779 to Hathaway; and 3,909,227 to Dickinson generally teach perforated members, e.g., screens, which are immersed into a pool of molten glass generally transverse to the direction of glass flow. The molten glass generally passes through the openings in the perforated members to screen out undesirable elements and to promote homogenization of the glass.
U.S. Pat. No. 3,265,485 to Carney et al. generally teaches a method of melting glass wherein glass flow patterns are controlled by internally cooling preselected areas of the pool with a plurality of elongated cooling elements. While thermal activity might be utilized to control the patterns of flow within a glass melting furnace, such a technique might not prove beneficial in removing gas inclusions. For example, to diminish the rate of flow of the minimum residence time flow 88 adjacent the surface of the pool by cooling might also create a high viscosity surface layer, or "skin", which would impede the rise of gas inclusions toward the surface.
Thus, while the teachings of each of the above-referenced patents may be advantageously practiced, there still remains a need in the glass-making art for a method whereby the pattern of glass flow within a glass melting furnace may be controlled to facilitate the removal of gas inclusions from the molten glass prior to their passing downstream into the final product.